One type of display system is a projection display system, where a projection lens is used to project the image to a screen. Movie theaters are one example of such display systems on a large scale. More recently, television sets and digital cinema systems have been developed that also use projection lenses. The projection lens may be for either front or rear projection, depending on whether the lens is on the viewer side of the screen or behind the screen.
Spatial light modulators (SLMs) are a type of display device that may use a projection lens. In general, SLMs are arrays of pixel-generating elements that emit or reflect light to the display screen via the projection lens. The SLM modulates light by turning the pixel-generating elements on or off.
An example of an SLM is a DMD (digital micro-mirror device). A DMD is an electromechanical device, whose pixel-generating elements form an array hundreds or thousands of tiny tilting mirrors. To permit the mirrors to tilt, each is attached to one or more hinges mounted on support posts, and spaced by means of an air gap over underlying control circuitry. The control circuitry provides electrostatic forces, which cause each mirror to selectively tilt. Incident light on the mirror array is reflected by the "on" mirrors in one direction and by the "off" mirrors in the other direction. The pattern of "on" versus "off" mirrors forms an image. In most applications, the light from the DMD is projected by a projection lens to a screen.
The size of the array of an SLM's pixel-generating elements determines the aspect ratio of the image it generates. For example, an SLM might have an array size of 1024.times.768, thereby generating images with an aspect ratio of 4:3. This 4:3 aspect ratio is consistent with NTSC television broadcast signals as well as those used for personal computer displays.
As display systems become more advanced, the variety of source data that they are capable of displaying has increased. Each type of source data may have its own format, that is, its own aspect ratio and. vertical and horizontal resolution. For example, one advance is the availability of "digital cinema", in which movie films are digitized for display. Movie films are characterized by a wide aspect ratio, such as 2:1. Another advance is high definition television with wide aspect ratios, such as the 16:9 format.
The obvious approach to using an SLM to display images having a desired aspect ratio is to use an SLM that has the same aspect ratio. Thus, a 2:1 image would be displayed with a 2:1 SLM. However, this approach requires a different SLM to be manufactured for each format having a different aspect ratio.
Another approach is to use an SLM that has a different aspect ratio but to forego use of the entire array. An example of this approach is the "letterbox" display, where a movie image is displayed with blank bands at the top and bottom. However, this approach is inefficient and the picture is distracting.
A third approach is to use an anamorphic lens to stretch the image to fit the desired aspect ratio. However, a problem with existing anamorphic lenses is that they are designed for a particular projection lens and are not likely to operate with a different projection lens. Also, existing anamorphic lenses are designed for large scale projectors where compactness and cost considerations are not overriding considerations.